Carrier wave modulation system



Aug. 30, 1938. N. H. CLOUGH ET AL 2,128,235

CARRIER WAVE MODULATION SYSTEM Filed Feb. 15, 1957 3 Sheets-Sheet l SOURCE OF MOM/14 7' /N6 I @MflDl/MTl/VG POEM/41$ INVENTORS N. H. Owl/6H AND BY ERNEST GREEN V WW ATT RNEY POTENTIALS Aug. 30, 1938- N. H. CLOUGH ET AL 2,128,235

- CARRIER WAVE MODULATION SYSTEM Filed Feb. 15, 1937 3 Sheets-Sheet D N M M My OHF. E TG {N V NUZR Ew .m m HM A MF. N V m %N m H 1 m mN Y M L B $1 SwkEEm a kfi fl .11 i usudkfiw v n Filed Feb. 15, 1937 5 Sheets-Sheet 5 N. H. CLOUGH ET AL CARRIER WAVE MODULATION SYSTEM Aug. 30, 1938.

Patented Aug. 30, 1938 NETE STATS mazes PATENT FFEQE 2,128,285 CARRIER WAVE MODULATION SYSTEM Application February 15, 1937, Serial No. 125,796 In GreatBritain February 14, 1936 20 Claims.

(Granted under the provisions of sec. 14, act of March 2, 1927; 357

This invention relates to carrier wave modulation systems of the so-called series modulation type, i. e. of the type wherein modulation is effected by varying the internal impedance of a tube or bank of tubes in series with a carrier frequency tube or bank of tubes across a source of potential.

As will be seen later the invention, in certain of its forms, is well adapted for use in, and is of considerable advantage when applied to, so-

called series modulation systems of the floating carrier type, that is to say, of the type wherein provision is made for varying the carrier power in the high frequency tube or tubes in accordance with the intensity of modulation. Series modulation systems of the floating carrier type are described in the specification of United States Patent #2,060,614 dated November 10, 1936 which corresponds to British Patent #427,026 of April In many cases it is desirable for well known practical reasons: to arrange the high frequency tube or bank of tubes in a series modulation sysstem on the negative instead of on the positive side of the modulating tube or bank of tubes and the present invention may be regarded as providing improved series modulation systems of this nature, i. e., wherein the modulating tube or bank of tubes is on the positive side.

According to this invention in a series modulation circuit arrangement wherein the modulating stage is on the positive side of the high or carrier frequency stage to be modulated instead of applying the modulating or other controlling potentials in the usual way directly across the input circuit of the said modulating stage, the said potentials are applied between the input electrode point of said stage and the negative or cathode side of the high frequency stage.

lIhe invention offers practical advantages which will be apparent to those skilled in the art among these advantages being that, since the modulation input circuit extends from the grid of the modulating tube or bank of tubes right across to the cathode point of the high frequency tube or bank of tubes, any undesired potential variations between grid and cathode of the modulating tube or bank of tubes (e. g. due to failure to obtain correct symmetry or balance where alternating current cathode heating is employed) will be of less adverse effect than would be the case were the modulating potentials app-lied as in the usual way. Another important advantage is that one end of the modulation input circuit is at anchored 5 (normally ground) potential, while, furthermore where a transformer is employed for applying the modulating potentials, the design of this transformer is simplified, because it need not be insulated with respect to ground.

An important subordinate feature of the inven 5 tion which may be adopted with advantage, particularly in floating carrier systems, consists in applying the modulating potentials to the grid of the modulating tube or bank of tubes from a point upon a direct current permeable impedance which is included in the anode circuit of a tube stage in cascade with and preceding the modulating stage.

In the preferred embodiments of this invention wherein floating carrier action is obtained, the said action is secured by applying potentials obtained by rectifying modulating potentials to the grid circuit of a stag-e preceding in cascade the modulating stage.

The present invention, though very advantage- 0 ously applicable to radio transmitters, is not limited to such transmitters, and for this reason and also for the sake of convenience in description, the carrier frequency tube or bank of tubes will be referred to in the matter which follows simply as the load resistance, though, in practice, of course, the said load resistance will generally be constituted by the anode-cathode space of a carrier frequency amplifier tube or bank of tubes.

The invention is illustrated in the accompanying drawings which show diagrammatically several embodiments. In the drawings, Figure 1 shows a series modulation system wherein the modulator is in the high potential end of the modulation circuit while the modulation amplifier cathodes are at the potential of the cathode of the high frequency stage.

Figures 2 and 3 are modifications of Figure 1 wherein an additional source of potential is insorted in the modulator circuit to facilitate 100% modulation.

In Figures 4 and 5 the number of modulators is increased and additional means is supplied for providing a bias for the final modulation ampli- I fier to cause the same to float, i. e., assume an operating potential which varies in accordance with signal amplitude.

Referring to Figure l the positive terminal +I-l'Il of a source (not shown) of anode potential is connected to the anode l of a main modulating tube 2 whose cathode 3 is connected through the high frequency stage (which is shown as comprising a pair of cross neutralized tubes 4, 5, driven by high frequency applied at 6 and having a tuned anode circuit 1) to the common negative terminal HTE and 2 (usually grounded) of the supply. Modulating potentials are applied across the primary 8 of a transformer 9 whose secondary l supplies voltage which is amplified by tubes l and i2 which are resistance capacity coupled in cascade and receive anode potential from +E-IT2. The output of tube i2 is fed to the grid 83 of a sub-modulator tube M whose anode is connected to +HTI through a resistance i a tap it upon which is connected either directly, or as shown through a damping resistance ii to the grid l8 of the main modulating tube 2. The cathode it of tube i4 is earthed and all three valve ll, l2, and it may receive fixed bias as indicated from terminals marked -GB. If, however, floating carrier action is required uni-directional potentials ob tained by rectifying modulating potentials may be superimposed on the fixed bias for the tube l4. By suitable choice of the value of resistance l5 and of the bias on tube M the main modulating tube 2 may be set to the correct working point. it will be seen that there is no voltage amplification between the anode of tube i i and the high frequency stage, but, in fact, a slight reduction of voitage. On the other hand, by proper choice of the value of 85. large power amplification can be obtained. The circuit 29 is coupled to the circuit l and supplies modulated output to any load represented merely by a resistance 2!.

The arrangement of Figure 1 has a weakness, namely that for 100% modulation the voltage across the high frequency stage must fall to zero and the cathode 3 of the tube 2 must accordingly come to earth potential at an instant when the grid E8 of the said tube 2 must be sufficiently negative to the said cathode (and therefore to ground) to give complete cut-off. The anode of the tube i i cannot however be brought below a definite minimum positive potential relative to its own cathode is. Figures 2 and 3 show modifications in which this weakness is avoided. In these figures the circuit is simplified diagrammatically by representing the high or carrier frequency stage merely as a resistance I-IFR and in order to simplify an understanding of the figures, the same reference (I-IFR) is marked on the chain line rectangle in which the high frequency stage is shown in Figure 1. Parts in Figure 2 corresponding to parts in Figure 1 are indicated by like references, as is the case throughout the figures. Further in Figures. 2 and 3 the modulating frequency amplifiers II and I2 are not shown.

The difference between Figures 1 and 2 is that in the latter figure a battery or other suitable bias source 22 is included in the connection between the grid l8 and the tap H5 in order that the tube 2 may receive the required extra bias. In Figure 3 the same result is achieved by connecting a bias source 25 between the cathode [9 of the tube M and ground. In the case of Figure 3 the tube i i, if of the filament type, must receive filament current from a suitably insulated filament supply; alternatively the tube i l may be of the indirectly heated cathode type. If the tubes 2 and i i are of the filament type and have their filaments energized from the same alternating current supply there will be to a considerable degree, cancellation of modulation due to A. C. filament heating, since, as respects this, the said tubes are in effect in opposition.

In many cases there will be, not merely one main modulating tube, but a number. Figure 4 shows a case where there are four main modulating tubes marked 2, 2', 2", 2" with their anodes tied together through so-called damping resistances 3i 3&3, 3D", 38" and their cathodes tied directly together, each grid l8, l8, 18" or Hi being connected through damping resistances H, il, i'l and i'i and negative bias sources 22,22, 22", 22' to taps l6, l6, l6" and IG on the resistance 65 in the anode circuit of the tube 14. The modulating amplifier stages preceding tube I4 are as in Figure 1 but in Figure 4 is shown a circuit for superimposing on the grid E3 of tube M, varying bias for floating carrier action. As this particular circuit forms per so no part of this invention it need not be described in detail. It will be sufficient to point out that modulating signal input, branched off from across the primary 8 of transformer 9 is fed via transformer 24, and regulating potentiometer 25 to an amplifier 26 the output from which is rectified by a full wave rectifier 21, the rectified resultant being amplified by the tubes 28 and 29 and superimposed in the grid circuit of tube M as shown. By suitable choice of the taps l6, l5, l6 and iii and of the Masses supplied from 22, 22, 22", 22" each tube l, l, I" and i' may be made to take an equal share of the D. C. feed and of the A. C. load. If the tubes 2, 2, 2 and 2" are matched, a common point on the resistance 15 may, of course, be used in place of the four individual grid circuit connections shown.

Figure 5 shows an arrangement very like that of Figure 4 but somewhat to be preferred. In Figure 5 the sources 22, 22', 22", 22" are dispensed with and required extra negative bias for the grids of tubes 2, 2, 2" and 2" obtained somewhat after the manner of Figure 3 by means of a suitable potential source 23a which maintains the cathode id of tube i l at a suitable negative potential with respect to ground. Further, the source 2311 is utilized also to supply anode potential for the tubes 23 and 29. These tubes, if of the filament type, should have their filaments energized from a suitable insulated source and the same source which also supplies the filament of tube M (if of the filament type) could be used for the purpose. In Figure 5 as in Figure 4 the tubes 2, 2, 2", 2" can be adjusted to take equal shares of D. C. feed and A. C. load.

Obviously any of the tubes referred to in the various embodiments may be replaced by a bank of tubes, and obviously also triodes may be replaced by any other convenient tubes, e. g. by pentodes.

The embodiments above described may be modified for telegraphy by arranging to apply telegraph signals instead of speech, television, or other modulating potentials, e. g. the grid of the tube is in (for example) Figure 5 could be arranged to be given either of two potentials in dependence upon the position of the telegraph key. In the case of telegraphy, of course, there would be no question of floating carrier action and the floating carrier bias supply circuits would, of course, be omitted.

Just as in the foregoing the floating carrier circuits may be omitted without afiecting the modulation circuits described, so the arrange ments described for floating carrier control may be used, if desired, without the modulation circuits. For example, in a telegraph transmitter the main modulation tubes might be retained and the telegraph carrier power of the station rendered continuously adjustable, the control being effected by varying the bias potential on the main modulators with the help of the fioating carrier arrangements described.

The apparatus at HFR need not be a radio transmitter but could be any known suitable load required to be supplied with modulated energy, e. g. a simple resistance.

We claim:-

1. In a modulation system, a high frequency relay of the electron discharge tube type having a cathode and another electrode coupled in a high frequency circuit in which wave energy to be modulated is caused to flow, a modulator tube having a cathode, an input electrode and an output electrode, means connecting the impedance between the cathode and output electrode of said modulator tube in series with the impedance between said electrodes in said high frequency relay tube, means for applying a direct current potential to the impedancesof said tubes in series, and means for applying modulating potentials between the input electrode of said modulator tube and the point at which negative potential is applied to said impedances in series.

2. In a modulation system, an electron discharge tube system having anode, cathode, and control grid electrodes coupled in alternating current circuits. wherein high frequency current to be modulated is caused to flow, a modulator tube having a control grid, a cathode, and an anode, means for connecting the cathode of said modulator tube to the anode of said discharge tube, means for connecting the cathode of said discharge tube to the negative terminal of a source of direct current potential and the anode of said modulator tube to the positive terminal of said source of direct current potential, and an impedance variab-le'at signal frequency connected in a circuit connection between the control grid of said modulator tube and the cathode of said electron discharge tube system.

3. In a modulation system, an electron discharge tube system having anode, cathode, and control grid electrodes coupled in alternating current circuits wherein high frequency current to be modulated is caused to flow, a modulator tube having a control grid, a cathode, and an anode, means for connecting the cathode of said modulator tube to the anode of said discharge tube, means for connecting the cathode of said discharge tube to the negative terminal of a source of direct current potential and the anode of said modulator tube to the positive terminal of said source of direct current, an impedance variable at signal frequency connected in a circuit in shunt to the impedances between the anode and cathodes of said tube and said device in series, and a connection between a point on said last circuit and the control grid of said modulator tube.

4. A system as recited in claim 3 wherein said last named connection includes an impedance.

5. In a modulation system, an electron discharge device having an anode, a cathode. and a control grid connected in alternating current circuits in which high frequency wave energy to be modulated is caused to flow, a modulator tube having an anode. a cathode, and a control grid, means connecting the cathode of said modulator tube to the anode of said electron discharge device, means connecting the cathode of said discharge device to one terminal of a high potential source and the anode of said modulator tube to the other terminal of said source, a modulation potential amplifier having an anode, a cathode, and a control grid, a substantially fixed impedance connecting the impedance between the anode and cathode of said modulation potential amplifier tube between the control grid of said modulator tube and the cathode of said electron discharge device, and means for applying modulating potentials to the control grid and cathode of said modulation potential amplifier tube.

6. A system as recited in claim 5 wherein said modulator tube is in the high potential end of said connection with said direct current potential source.

7. A system as recited in claim 1 wherein said modulator tube impedance is in the high potential end of said direct current circuit.

8. In a modulation system, an electron discharge device having an anode, a cathode, and a control grid connected in alternating current circuits: in which high frequency wave energy to be modulated is caused to flow, a modulator tube having an anode, a cathode, and a control grid, means connecting the cathode of said modulator tube to the anode of said electron discharge device, means connecting the cathode of said discharge device to one terminal of a high potential source and the anode of said modulator tube to the other terminal of said source, a modulation potential amplifier tube having an anode, a cathode, and a control grid, a substantially fixed impedance connecting the impedance between the anode and cathode of said modulation potential amplifier tube in shunt to the impedances between the anodes and cathodes of said electron discharge device and modulator tube in series, means connecting a point on said substantially fiXed impedance tothe controlgrid of said modulator tube, and means for applying modulating potentials to the control grid and cathode of said modulation potential amplifier tube.

9. In a modulation system, an electron discharge tube having anode, cathode, and control grid electrodes connected in alternating current circuits in which high frequency current to be modulated is caused to flow, a modulator tube having output electrodes connected in series with the impedance between the anode and cathode of said electron discharge tube and with a source of direct current potential, said modulator tube having an input electrode, a modulation potential amplifier tube having output electrodes coupled between the input electrode of said modulator tube and the cathode of said electron discharge tube, said modulation potential amplifier tube having a control grid, a source of modulating potentials connected with said control grid, and means including a rectifier having its output coupled with said control grid and its input coupled with said source of modulating potentials to control the operating point of said modulation potential amplifier in accordance with the amplitude of said modulating potentials.

10. In a modulation system, an electron discharge tube having anode, cathode, and control grid electrodes coupled in alternating current circuits wherein high frequency wave energy to be modulated is caused to flow, a modulator tube having output electrodes connected in series with the impedances. between the anode and cathode of said electron discharge tube, said modulator tube having an input electrode, means for applying direct current potential in shunt to said series connection, a fixed impedance and an impedance variable at signal frequency connected in shunt to said series connection, and a resistance connecting a point on said fixed impedance to the input electrode of said modulator tube.

11. A system as recited in claim 10 wherein said last named connection includes a source of direct current potential having its negative ter minal adjacent the input electrode of said modulator tube and its positive terminal adjacent said fixed impedance.

12. A system as recited in claim 10 wherein said last named connection includes a resistance and a source of potential in series with the negative terminal of said source of potential adjacent the input electrode of said modulator tube and the positive terminal of said source of potential adjacent said fixed impedance.

13. A system as recited in claim 10 wherein said last named connection includes a resistance.

i l. In a modulation system a high frequency wave amplifier of the discharge tube type having an anode and a cathode and having alternating current circuits wherein high frequency wave energy to be modulated is caused to flow, a plurality of modulator tubes each having an anode, a cathode, and a control grid, means for connecting the impedances between the anodes and cathodes of said modulator tubes in series with the impedance between the anode and cathode of said electron discharge tube and means for connecting the said series connection in shunt to a source of direct current potential, an impedance variable at signal frequency in series with a fixed impedance in shunt to said aforesaid tube impedances, and a separate connection between the control grid of each of said modulator tubes and points on said fixed impedance.

15. A system as recited in claim 14 wherein each of said last named connections includes a source of potential having its negative terminal adjacent the control grids of said modulator tubes and its positive terminals adjacent said fixed impedance.

16. A system as recited in claim 14 wherein each of said last named connections includes a resistance.

17. A system as recited in claim 14 wherein each of said last named connections includes a resistance and a source of potentials in series, with the negative terminals of said sources of potential adjacent the control electrodes of said tubes and the positive terminals of said sources of potential adjacent said fixed impedance.

18. A system as recited in claim 14 wherein said impedance variable at signal frequency, comprises the anode to cathode impedance of an electron discharge tube having a control grid coupled to a source of modulating potentials and wherein a rectifier is coupled to said source of modulating potentials and to said control grid of said last named tube to control the potential thereof in accordance with the rectified modulating potentials.

19. In a modulation system, an electron discharge tube having an anode and a cathode and having alternating current circuits wherein high frequency wave energy to be modulated is caused to flow, a plurality of modulator tubes each having an anode, a cathode, and a control grid, means connecting the impedance between the anode and cathode of said discharge tube in series with the impedance between the anodes and cathodes of said modulator tubes, means connecting the cathode of said discharge tube to the negative terminal of a source of direct current potential, means connecting the anodes of said modulator tubes to the positive terminal of said source of direct current potential, a modulation potential amplifier tube having an anodega cathode, and a control grid, a source of potential connecting the cathode of said modulation potential amplifier tube to the cathode of said electron discharge tube, an impedance connecting the anode of said modulation potential amplifier tube to the anodes of said modulator tubes, separate connections between the control grids of said modulator tubes and said last named impedance, a source of modulating potentials, amplifying means coupling said source of modulating potentials to lation potential amplifier tube, and rectifying means coupling said source of modulating potentials to the control grid of said modulation po' tential amplifier tube.

20. A modulation system as recited in Claim 2 wherein said impedance variable at signal frequency connected in a circuit connection between the control grid of said modulator tube and the cathode of said electron discharge tube system, comprises an-additional electron discharge tube having its anode connected to the control grid of said modulator tube and its cathode connected to the negative terminal of a source of potential the positive terminal of which is connected to the cathode of said electron discharge tube system.

NEWSOME HENRY CLOUGI-I. ERNEST GREEN.

the control grid of said modu- 

